The invention relates to a method for reducing errors of a rotary device, which comprises a first part and a second part which is rotatable relative to the first part about an axis of rotation of the rotary device, and a rotational position measuring device for measuring rotational positions of the first part and the second part relative to one another, wherein the rotational position measuring device comprises a rotational position sensor and a measurement body interacting with the rotational position sensor for measuring the rotational position, and wherein the rotational position sensor is connected to the first part and the measurement body is connected to the second part, or vice versa. The invention furthermore relates to an arrangement by means of which the method is implementable. Here, the assumption is made that the errors of the rotary device are reproducible, at least in part.
The invention moreover relates to a method for reducing errors of a rotary device when determining coordinates of a workpiece or when machining a workpiece. The rotary device enables a rotational movement of the workpiece about an axis of rotation of the rotary device while determining the coordinates or while machining the workpiece. The invention furthermore relates to an arrangement by means of which the method is implementable. Here, the assumption is made that the errors of the rotary device are reproducible, at least in part.
It is common practice to rotatably bear workpieces for the purposes of measuring the coordinates thereof or for the purposes of machining the workpiece. By way of example, workpieces are arranged on rotatable tables (so called rotary tables) in the field of coordinate metrology. In this way, the workpiece can be brought into various work alignments, in which the coordinate measuring machine operates, i.e. measures coordinates of the workpiece. In particular, the coordinates of the workpiece can be measured continuously (e.g. in scanning fashion) while the rotary device rotates the workpiece about the axis of rotation thereof.
Corresponding statements apply to the machining of a workpiece by a machine tool. The workpiece can be brought into various work alignments in order to machine the workpiece. In particular, the workpiece can be rotated continuously while it is being machined.
In particular, the work alignment can be defined by a direction that extends perpendicular to the axis of rotation and through a point on the surface of the workpiece, at which the workpiece is sensed or at which the workpiece is machined. Therefore, the force acting on the workpiece during the tactile probing of the workpiece with a probe or during the machining of the workpiece can act, in particular, perpendicular to the axis of rotation in the direction of the work alignment.
In the field of coordinate metrology, for checking the shape of a workpiece, it is often advantageous to sense the workpiece with a probe which has an almost constant work alignment and work position relative to the rotary device while the rotary device rotates the workpiece. The work position and work alignment are not entirely constant, since the workpiece is generally not arranged exactly rotationally symmetrically with respect to the axis of rotation of the rotary device and/or is not, or not exactly, shaped rotationally symmetrically. By way of example, a probe of a coordinate measuring machine, which probes the surface of the workpiece in a tactile fashion, may be held by the coordinate measuring machine in a fixed position and with a fixed alignment, the probe being deflected to a different extent relative to a holder of the probe, depending on the workpiece shape to be measured. Owing to the almost constant work alignment and work position, errors of the coordinate measurement due to position-dependent and alignment-dependent errors of the coordinate measuring machine can be minimized. The errors of the rotary device in this case crucially determine the measurement result. The speed of the measurement of the workpiece can thereby be increased in many cases.
A special measuring problem in the field of coordinate metrology consists in the waviness analysis when checking the shape, particularly of rotationally symmetric areas of workpieces. The deviations of the actual shape from the ideally rotationally symmetric one often exhibit a wavy profile. However, the movement error of the rotary device, by which the real rotational movement of the rotary device deviates from an ideal rotational movement, can lead to the results of the waviness analysis being particularly inaccurate, in particular more inaccurate than results when measuring coordinates of individual surface points of a workpiece.
Corresponding statements apply when machining a workpiece by a machine tool. In a disadvantageous case, waves with particularly large amplitudes may result due to the movement errors of the rotary device, by means of which the workpiece is rotated during the machining, during the sought-after production of a rotationally symmetric area.
In order to reduce the errors of the rotary device, the rotary device may be designed in such a way that the error meets specifications. In particular, it is possible to use air bearings for mounting the rotationally mobile parts of the rotary device, and in the case of motor-driven rotary devices it is possible to use direct drives. The smaller the error of the rotary device is intended to be, the higher is the design outlay.
As an alternative or in addition, errors of the rotary device may be measured with a coordinate measuring machine, a calibration body or an arrangement of calibration bodies being arranged on the rotatable part of the rotary device (for example placed on the rotary table) and measured. Measurement of the errors of the rotary device with respect to all six possible degrees of freedom of the movement using a coordinate measuring machine which can also measure workpieces is, however, time-consuming. If a high accuracy is required, the calibration needs to be repeated, for example when the rotary device is subjected to temperature variations. Corresponding considerations apply for a rotary device which is configured in order to hold workpieces rotatably in the machining range of a machine tool. The outlay for calibration is then usually even greater compared with coordinate metrology, since in the field of coordinate metrology the coordinate measurement machine which subsequently carries out the measurement of workpieces can mostly also be used for the calibration.
Eric Marsh describes in “Precision Spindle Metrology”, ISBN 978-1-932078-77-0, in particular Chapter 2, concepts for the description of movement errors of a precision spindle.
WO 2013/007285 A1 discloses an arrangement for measuring coordinates of a workpiece and/or for machining the workpiece, wherein the arrangement comprises a first part and a second part movable relative to the first part, wherein the relative movability of the parts is provided in addition to possible movability of a probe optionally additionally fastened to the arrangement, which movability when mechanically sensing the workpiece for the purposes of measuring the coordinates is given by a deflection of the probe from a neutral position, wherein a measurement body is arranged at the first or second part and at least one sensor is arranged at the other part, i.e. at the second or first part, wherein the sensor is configured to generate a measurement signal in accordance with a position of the measurement body and hence in accordance with the relative position of the first and second part.
WO 2013/007286 A1 discloses a method for calibrating a measurement arrangement for determining rotational positions of a rotary device, which has a first part and a second part rotatable relative to the first part about an axis of rotation.
DE 199 07 326 A1 discloses an angle measuring system for highly precise determination of the angular position of an object rotating about an axis of rotation.
WO 2011/064317 A2 discloses a calibration method, implementable without reference system, for an angle measuring apparatus.
EP 1 498691 A1 discloses a method for correcting the measurement results of a coordinate measuring machine, in which a workpiece is sensed continuously, comprising the following method steps: determining the dynamic bending behavior of the probe as a one-dimensional or multidimensional parameter field, in particular as a dynamics tensor, calculating correction values from the parameter field taking into account the acceleration of the probe, correcting the measurement results with the correction values, wherein the parameter field describes the deviations in the case of acceleration of the probe normally in relation to the workpiece surface.
DE 100 06 753 A1 discloses a rotary/swivel apparatus for probe heads of coordinate measuring machines, comprising at least two rotary joints for aligning the probe heads in terms of angle.
In the field of coordinate metrology, a high precision of rotary devices is required, for example in the so-called roundness check, in which circular contours of workpieces (e.g. the circumferential faces of cylinders) are intended to be checked for deviations from the ideal circular form.
In another case, pitch measurements (i.e. angular distances in a circumferential direction around an axis of rotational symmetry) are intended to be performed in conjunction with the use of rotary devices, e.g. the angular distances of the teeth or flanks on toothed wheels. To this end, very precisely measuring rotational position measuring devices of the rotary device are required, because, otherwise, an equally large measurement error emerges purely due to the error in the rotational position measuring device. However, very precisely measuring rotational position measuring devices are required in other cases in the field of coordinate metrology or of machine tools.
In the case of such a rotational position measuring devices, the so-called eccentric error occurs and supplies a large error contribution. The overall eccentric error is substantially composed of the eccentric error of the rotary bearing for bearing the rotational movement of the rotary device and the assembly error of the rotational position measuring device. It is common practice to largely eliminate the overall eccentric error by adjustment. As a result, the residual error contribution of the overall eccentric error to the overall error of the rotary device can be made to be very small. A further option for reducing the eccentric error consists of respectively arranging a rotational position sensor at positions lying opposite one another in respect of the axis of rotation and of combining the measurement results thereof with one another by calculation in such a way that the eccentric error is eliminated or calculated and subsequently able to be used for correction purposes.
A rotational position measuring device in the field of coordinate metrology often comprises a measurement body, which has a multiplicity of marks distributed about the axis of rotation. The measurement principle is based on the marks subdividing a revolution around the axis of rotation into angular portions with the same size. Deviations from this uniform arrangement of the marks are therefore referred to as graduation errors. Determining the graduation error by separate measurement is possible with corresponding outlay. Once a reference angular position of the measurement body has been determined, the graduation error can be corrected by computation.
In order to reduce the aforementioned eccentric errors from the outset, it is possible to use complicated rotary bearings, which merely bring with them a small eccentric error, for bearing the rotational movement of the rotary device. The outlay for such rotary bearings is high and, nevertheless, generally requires an adjustment in order to further reduce the eccentric error.
If there is less outlay for the rotary bearing of the rotary device, not only is it necessary accept a comparatively large eccentric error but additional movement errors also occur, i.e. unwanted translational and rotational movement components during the rotational movement of the rotary device. In principle, all movement components by which a real rotational movement differs from an ideal rotational movement of the rotary device are unwanted. In the case of an ideal rotational movement, all regions of the rotatable part of the rotary device rotate with the radial position of the region remaining constant in respect of the rotational axis, if a rotational movement occurs. As a result, the axis of rotation of the rotary device is stationary, i.e. it does not carry out translational movements in the direction of the axis of rotation or transverse to the axis of rotation and it does not tilt either. A tilt of the axis of rotation is tantamount to a change in the alignment of the axis of rotation.
Movement errors of the rotary device can be measured and corrected by calculation during a subsequent operation of the rotary device. However, the outlay for the computational correction is very high, in particular because the movement errors often have very small amplitudes. In particular, a correction at the location at which the rotational position measuring device of the rotary device measures the rotational position is very complicated since the movement error merely leads to a rotational position measuring error in the region of fractions of arc seconds. Moreover, known methods for determining the rotational position errors require measurements which may take several hours. Moreover, the temperature can change during this long measurement duration, which in turn leads to a change in the movement error.
A further option for reducing the effects of the movement error on the rotational position measurement consists of using so-called dedicated bearings. These allow the components, arranged at the parts of the rotary device rotatable relative to one another, of the rotational position measuring device (e.g. rotational position sensor and measurement body with marks) to implement the movement errors of the rotary device together. Such dedicated bearings are particularly demanding.
The rotational position measuring error has a particularly strong effect on the error when measuring the coordinates of the workpiece by a coordinate measuring machine or on the machining of the workpiece by a machine tool, particularly if workpieces which have at least regions arranged at a large distance from the axis of rotation of the rotary device are intended to be rotated by a rotary device. If the controller of the coordinate measuring machine or of the machine tool assumes an incorrect rotational position, this has an effect on the coordinate measuring error or on the machining which is proportional to the radial distance. By way of example, rotational position measuring errors of up to one arc second occur in practice, leading to a position error of the workpiece in a direction tangential to the circumferential direction of approximately one micrometer in the case of a radial distance of 0.2 m.